<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
    <style type="text/css">
        .row div {
            border: 1px solid red;
            margin: 10px 0;
            padding: 0;
        }
    </style>
    <title></title>
</head>
<body>
<div class="container">
    <div class="row">
        <div class="col-xs-12 col-md-8">.col-xs-12 .col-sm-6 .col-md-8</div>
        <div class="col-md-4">.col-xs-6 .col-md-4</div>
    </div>
    <div class="row">
        <div class="col-xs-6 col-sm-4">.col-xs-6 .col-sm-4</div>
        <div class="col-xs-6 col-sm-4">.col-xs-6 .col-sm-4</div>
        <!-- Optional: clear the XS cols if their content doesn't match in height -->
        <div class="clearfix visible-xs"></div>
        <div class="col-xs-6 col-sm-4">.col-xs-6 .col-sm-4</div>
    </div>
</div>
</body>
</html>